Two major applications of interferometers are in metrication, to measure differences in path length, and in remote sensing, to determine the spectral content of light. In a typical metrication application, a laser beam is split into two beam components at a beamsplitter surface, the components pass along two different paths to two different optical devices such as retroreflectors that return them back to the beamsplitter surface where they are combined, and the combined beam components are directed onto a light detector that detects interference patterns. The detected interference patterns indicate slight differences in the path lengths of the beam components, to thereby detect slight movements of the retroreflectors relative to one another. In a typical remote sensing application, light of an unknown spectrum, such as from stars, the sun, or laboratory sources, is combined with light of known wavelengths such as from a laser, and the interference pattern established between the known and unknown light is detected to determine the spectral composition of the unknown source.
Simple interferometer systems have been very sensitive to misalignments of the beamsplitter devices. Interferometer systems have been proposed which minimized such sensitivity, but they have required a larger number of relatively complex and heavy components. For example, U.S. Pat. No. 3,109,049 by Williams shows a tilt compensated interferometer, which requires four retroreflectors and a multicube optical device which would have to be fairly heavy to accommodate a wide beam. In a variety of applications such as for in flight applications, it is desirable to utilize a relatively simple and low weight system.